1. Field of the Invention
The present invention generally relates to an evaporator for cooling apparatus used in refrigerating cycle, and more particularly to an evaporator for cooling apparatus in which a plurality of refrigerant flow passages are connected in parallel with each other.
2. Related Art
A conventional apparatus of this kind of the evaporator for cooling apparatus includes an evaporating portion in which inflow passages and outflow passages are connected in parallel to each other with a plurality of refrigerant flow passages, a heat exchanging portion for heat exchanging between cooled flow passages which lead to a pressure reducing valve in a refrigerating cycle and cooling flow passages which lead to outflow passages for introducing the refrigerant into an outlet, pressure reducing means for reducing the refrigerant pressure within the cooled flow passages and introducing the refrigerant into the inflow passages, and a bypass flow passage for bypassing the heat exchanging portion and the pressure reducing means for introducing the refrigerant into the inflow passages in the evaporating portion.
In this evaporator for cooling apparatus, after the pressure of the refrigerant having been condensed by a condenser in the refrigerating cycle is reduced by a pressure reducing valve, the refrigerant is further cooled in the heat exchanging portion. Then, the pressure of the refrigerant is further reduced by the pressure reducing means, and the refrigerant evaporates in the evaporating portion. The refrigerant is introduced into the cooling flow passage in the heat exchanging portion while absorbing evaporation heat from the ambient air. The temperature of the refrigerant having been introduced into the cooling passages is lower than that of the refrigerant within the cooled flow passages. Therefore, such refrigerant absorbs the heat of the refrigerant within the cooled flow passages of heat and is returned into the refrigerating cycle. In this way, in this kind of the evaporator for cooling apparatus, the dryness of the refrigerant to be introduced into the evaporating portion (the ratio of the gas component of the refrigerant) can be reduced and thereby heat exchange efficiency can be improved by providing the heat exchanging portion (so-called "super cool").
Also in this kind of the evaporator for cooling apparatus includes the bypass flow passage bypassing the heat exchanging portion and the pressure reducing means for introducing the refrigerant into the inflow passages in the evaporating portion, thereby the following effects can be obtained. That is to say, when the refrigerant pressure is low at the upstream side of the pressure reducing valve, e.g. when the temperature is low such as in the winter season or when the cooling apparatus is in a trial operation, the temperature of the refrigerant within the cooled flow passages in the heat exchanging portion falls to or below the temperature of the refrigerant within the cooling flow passage; in such a case, so-called reverse heat exchange occurs, i.e., the refrigerant within the cooled flow passages is heated by the refrigerant within the cooling passages; then, the gasification of the refrigerant within the cooled flow passages is facilitated, and it becomes difficult for the refrigerant to flow through the heat exchanging portion; at this time, the refrigerant passed through the bypass flow passage can reach the evaporating portion without the reverse heat exchange; for this reason, as described above, even if the refrigerant pressure at the upstream side of the pressure reducing valve is low, the heat exchange efficiency can be maintained.
Furthermore, another conventional apparatus of this kind of an evaporator for cooling apparatus, as disclosed in the Japanese Unexamined Patent Publication No. 6-185831, includes a valve element which opens when the refrigerant pressure at the upstream side of the pressure reducing valve falls within the bypass flow passage. In this apparatus, when the refrigerant pressure at the upstream of the pressure reducing valve is high enough to prevent the reverse heat exchange, the refrigerant is introduced into the heat exchanging portion, the valve element may close the bypass flow passage to introduce the whole quantity of the refrigerant into the heat exchanging portion. This can further lower the refrigerant dryness and further improve the heat exchange efficiency. Moreover, when the refrigerant pressure at the upstream of the pressure reducing valve is so low, the reverse heat exchange may occur and the valve element opens the bypass flow passage and prevents the reverse heat exchange.
However, if the valve element opens only when the refrigerant pressure at the upstream side of the pressure reducing valve excessively falls, there is a possibility that the bypass flow passage does not open even if the reverse heat exchange occurs in the heat exchanging portion and the heat exchange efficiency of the evaporator for cooling apparatus may be deteriorated. On the other hand, if the valve element is arranged to open when the refrigerant pressure at the upstream side of the pressure reducing valve slightly falls, the following problem occurs. That is to say, the dryness of the refrigerant to be introduced into the evaporating portion through the bypass flow passages is higher than that of dryness of the refrigerant to be introduced into the evaporating portion through the heat exchanging portion unless the reverse heat exchange occurs. This may raise the dryness of the refrigerant to be guided into the evaporating portion, and there is a possibility that the sufficiently uniform supply of the refrigerant into each refrigerant flow passage in the evaporating portion can not be achieved. If this is the case, the sufficient improvement in the heat exchange efficiency in the evaporating portion can not be achieved.